As mobile devices have been continuously developed and the demand for such mobile devices has increased, the demand for secondary batteries has also sharply increased as an energy source for such mobile devices. Accordingly, much research into batteries satisfying various needs has been carried out.
Typically, in terms of the shape of batteries, the demand for prismatic secondary batteries or pouch-shaped secondary batteries that are thin enough to be applied to products, such as cellular phones, is very high. In terms of the material for batteries, on the other hand, the demand for lithium secondary batteries, such as lithium ion batteries and lithium ion polymer batteries, which exhibit high energy density, discharge voltage, and output stability, is also very high.
In addition, secondary batteries may be classified based on the shape of a battery case of each of the secondary batteries into a cylindrical battery, configured to have a structure in which an electrode assembly is mounted in a cylindrical metal container, a prismatic battery, configured to have a structure in which an electrode assembly is mounted in a prismatic metal container, and a pouch-shaped battery, configured to have a structure in which an electrode assembly is mounted in a pouch-shaped case made of a laminated aluminum sheet.
Particularly, in recent years, a lot of interest has been directed to a pouch-shaped battery configured to have a structure in which a stacked or stacked/folded type electrode assembly is mounted in a pouch-shaped battery case made of a laminated aluminum sheet because of low manufacturing costs, light weight, easy modification of the shape thereof, etc. In addition, the use of such a pouch-shaped battery has gradually increased.
Furthermore, secondary batteries may be classified based on the structure of an electrode assembly, which has a structure in which a positive electrode and a negative electrode are stacked in the state in which a separator is interposed between the positive electrode and the negative electrode. Typically, the electrode assembly may be configured to have a jelly-roll (wound) type structure in which a long sheet type positive electrode and a long sheet type negative electrode are wound in the state in which a separator is disposed between the positive electrode and the negative electrode or a stacked type structure in which a plurality of positive electrodes and a plurality of negative electrodes, each of which has a predetermined size, are sequentially stacked in the state in which a plurality of separators is disposed respectively between the positive electrodes and the negative electrodes. In recent years, in order to solve problems with the jelly-roll type electrode assembly and the stacked type electrode assembly, there has been developed a stacked/folded type electrode assembly, which is a combination of the jelly roll type electrode assembly and the stacked type electrode assembly, having an improved structure in which a predetermined number of positive electrodes and a predetermined number of negative electrodes are sequentially stacked in the state in which a predetermined number of separators are disposed respectively between the positive electrodes and the negative electrodes to constitute a unit cell, after which a plurality of unit cells is sequentially folded in the state of being placed on a separation film.
FIG. 1 is a view schematically showing a general structure of a conventional representative stacked/folded type electrode assembly.
Referring to FIG. 1, an electrode assembly 100 includes a combination of unit cells 110, 130, 150, and 170, each of which is configured to have a structure in which a negative electrode 101, a separator 103, a positive electrode 102, another separator 103, and another negative electrode 101 are sequentially stacked, and unit cells 120, 140, 160, and 180, each of which is configured to have a structure in which a positive electrode 102, a separator 103, a negative electrode 101, another separator 103, and another positive electrode 102 are sequentially stacked. A separation film 190, which is interposed between the unit cells 110, 120, 130, 140, 150, 160, 170, and 180 of the electrode assembly 100, surrounds side surfaces of the unit cells 110, 120, 130, 140, 150, 160, 170, and 180 at which no electrode terminals are formed.
The electrode assembly 100 is manufactured by winding the separation film 190 in the state in which the unit cells 110, 120, 130, 140, 150, 160, 170, and 180 are arranged on the separation film 190. Consequently, the electrode assembly 100 includes a total of 12 negative electrodes 101, a total of 12 positive electrodes 102, a total of 16 separators 103, and one separation film 190.
In the case in which electrodes having the same polarity are located at opposite ends of each of the unit cells, however, the electrode assembly has a vertically asymmetrical structure, since electrodes having different polarities are located at opposite ends of the electrode assembly, i.e. the outermost electrodes of the electrode assembly are constituted by a positive electrode and a negative electrode. In the case in which the electrode assembly has a vertically asymmetrical structure, the results of nail penetration tests at the opposite ends of the electrode assembly are different from each other, with the result that the safety of a battery cell including the electrode assembly may be reduced. In addition, it is necessary to modify the structure of a battery case, into which the electrode assembly is inserted, depending on the direction in which the electrode assembly is inserted into the battery case, since the external appearance of the electrode assembly is asymmetrical.
Furthermore, active materials are applied to opposite surfaces of a current collector that constitutes each of the outermost electrodes of the electrode assembly. When the battery cell is broken by a conductive material, such as a metallic material, therefore, the active materials come into direct contact with the conductive material, with the result that a short circuit may occur in the battery cell, whereby the battery cell may catch fire or explode.
Therefore, there is a high necessity for a battery cell configured to have a structure in which the outermost electrodes of an electrode assembly are constituted by electrodes having the same polarity in the case in which an even number of unit cells are used, with the result that the electrode assembly can be inserted into a battery case irrespective of the direction in which the electrode assembly is inserted into a battery case, the external appearance of the electrode assembly is symmetrical, and the results of nail penetration tests at the opposite ends of the electrode assembly are the same.